Envoy jerked forward when started...

[Denali n DOO]

My wife is trying to explain to me what happened to day with the Envoy . I tried a few search words and found nothing. She was parked in a store parking lot, she says it was on a good incline with the front end up higher than the back. When she turned the key to start it, the truck jerked forward and then back, she heard a deep sounding bang at the same time. The engine did start as normal, no delay or nothing. Shifted in to reverse and drive just fine. I'm gonna go for a drive and try park on a few incline's and see if I can get it to happen. I've never experienced or even heard of such a thing happening.

Any ideas as to what might have happened to cause the jerk motion forward then backward when started, it must be something major to move the whole truck?

 

[MacMan]

This has been reported for quite a while on this and "other" forums. My '02 does it once in a while....no idea why, but doesn't seem to have hurt anything since I have 195k on it. Only difference is that I never hear any "deep sounding bang".

 

[Denali n DOO]

This has been reported for quite a while on this and "other" forums. My '02 does it once in a while....no idea why, but doesn't seem to have hurt anything since I have 195k on it. Only difference is that I never hear any "deep sounding bang".

Any threads here that you recall? I tried a search.

Thanks.

 

[The_Roadie]

I'm not remembering anything but the "feeling of being bumped when stopped" threads that (mostly, not always) end up being related to driveshaft slip yoke stickiness at the transmission or transfer case tailpiece. Those incidents are relatively gentle bump feelings, never accompanied by a bang.

Was she parked using the transmission park pawl only? Not the ebrake? Definitely a bad idea to trust the parking pawl only to keep it from rolling on a hill.

Was she in anything other than 2HI mode?

Also I'd check the transmission fluid level in case something odd happens when it's not level.

 

[Denali n DOO]

I'm not remembering anything but the "feeling of being bumped when stopped" threads that (mostly, not always) end up being related to driveshaft slip yoke stickiness at the transmission or transfer case tailpiece. Those incidents are relatively gentle bump feelings, never accompanied by a bang.

Was she parked using the transmission park pawl only? Not the ebrake? Definitely a bad idea to trust the parking pawl only to keep it from rolling on a hill.

Was she in anything other than 2HI mode?

Also I'd check the transmission fluid level in case something odd happens when it's not level.

She is familiar with "feeling of bumped when stopped" and it's not that, we had a new shaft installed for that, and it hasn't happened again since, touch wood. The deep bang part may be coincidence as I have heard a deep thud once before after the truck was running, I thought that was something in the HVAC ducting, like a door slamming. She was parked using the transmission park pawl only. She didn't change 4x4 settings, it is in 2 hi. The only thing done recently was new fluid in front and rear differentials and a new pinion seal in front differential which showed signs of sweating and possible leaking. I tried to duplicate it but wasn't able too. I just thought of a really steep hill near here so I'm gonna go try again. I'll check the transmission fluid level as well.

 

[The_Roadie]

Did you do transfer case fluid recently?

 

[Denali n DOO]

Did you do transfer case fluid recently?

Last transfer case fluid was about 25,000 kms ago...

 

[fletch09]

This has been reported for quite a while on this and "other" forums. My '02 does it once in a while....no idea why, but doesn't seem to have hurt anything since I have 195k on it. Only difference is that I never hear any "deep sounding bang".

I too, every once in a while, experience a lurch forward when starting. sitting on level ground in the garage. never heard a bang though.
maybe since it was on an incline, is why the "bang" occured. parking prawl catching again

 

[Chickenhawk]

There is no problem. It is a common occurrence, and it happens to me a couple times a year.

It is simply residual fluid still in the clutch packs.

There is a GM technical service bulletin on it:

Some vehicles may rock or move slightly forward or rearward while in park at start up after cold soak. This condition may be accompanied by a clunk noise. This is a slight movement that is more often seen visually, rather than felt, when viewed from the outside and using the auto-start feature, if equipped.
This slight movement is due to the residual transmission fluid in the clutch packs that creates a partial apply on start up. This movement is stopped by the parking pawl and/or transmission fluid exhausting completely from the clutch packs.
This slight movement has no adverse affects on the transmission and no repair should be attempted. This condition is normal. This can be verified by comparing with another identically built vehicle under the same cold conditions.
To prevent this condition from occurring, the parking brake should be utilized as per the Owner's Manual recommendations anytime the vehicle is parked.

 

[Sparky]

Mine did that once. I guess I need to yank the rear wheels and check the parking brakes (driver side gets stuck on if I try to use it).